Hitherto, heat-sensitive recording papers have been used as a heat-sensitive recording type, self-adhesive label.
In the case of such a heat-sensitive recording type, self-adhesive label, when plasticizers present in plastic films, such as dioctyl adipate (DOA) and dioctyl phthalate (DOP), penetrate into a heat-sensitive, color-forming layer, there was found a problem of decolorization of the printed portion (color-formed portion). Accordingly, it cannot be used as a self-adhesive label for plasticizer-containing plastic wrapping film. Further, when water or oils penetrate into the heat-sensitive, color-forming layer, the printed image smears or the image density reduces with a lapse of time. Moreover, in the case that the printing is carried out by bringing the heat-sensitive, color-forming layer into direct contact with a thermal head, if the amount of a binder present in the heat-sensitive, color-forming layer is too high, the residue build-up and sticking readily occur. Accordingly, in order to prevent such occurrence, the amount of the binder in the heat-sensitive, color-forming layer is necessarily reduced, which, however, leads to a drawback that the surface strength is weak. Thus, in the case that the printing is carried out on the surface of the heat-sensitive, color-forming layer, the heat-sensitive, color-forming layer is entrained in the rubber roll side, whereby the printing is no longer possible.
In order to remove the above drawbacks, an attempt to not only prevent the transfer and penetration of plasticizers, water and oils into the heat-sensitive, color-forming layer but also impart it with a surface strength has hitherto been made by providing a protective layer on the heat-sensitive, color-forming layer for the purpose of adapting such a heat-sensitive recording material to self-adhesive labels.
As a main component of the protective layer, there are used water-soluble high molecular weight compounds such as, for example, polyvinyl alcohol or modified products thereof, starch or modified products thereof, etc.
However, in the case of the conventional heat-sensitive recording type, self-adhesive labels having a protective layer mainly composed of a water-soluble high molecular weight compound, though the decolorization of the printed portion by plasticizers and the surface strength are improved, there may occur the transfer and penetration of water and oils from the edge portions and back side of the label, resulting in decolorization of the printed portion by water and oils. Further, by providing the protective layer on the heat-sensitive, color-forming layer, when the color formation is carried out by means of a thermal head, there occur problems that the heat responsibility reduces and the color formation sensitivity lowers. Still further, because of the increase of a production step for providing the protective layer, there is a disadvantage of cost increase.